Mastery Gems are found almost entirely through monster loot. No vendor in Britannia sells Mastery Gems – you earn them from drops. The necklace itself is a crafted item.
The tougher the monster, the better the gem it can drop:
Drop chances are intentionally kept low and are not published – treat any Mastery Gem drop as a nice surprise, not something to farm on a schedule. Tougher and rarer content is simply more likely to reward one over time.
Boss-tier gem drops require more than one player, each doing a real share of the damage, to have taken part in the kill. A single player running multiple accounts on their own won’t farm boss gems this way.
The Halloween event adds several named bosses to the mini-boss gem ladder, each with the same chance at a Topaz gem as any other named mini-boss:
None of these are guaranteed to drop a gem – they simply share the same odds as the rest of the mini-boss ladder.
Completing a nightly hunt contract from the Bounty Board gives the contract holder (not assisting group members) a small extra chance at a bonus Ruby gem, on top of anything the contract’s target monster would normally drop. Harder contracts have a somewhat better chance than easier ones. Bounty Board gems count toward the same daily household limit as the board’s other item rewards.
Three Halloween quest lines don’t reward gems directly, but each grant a one-time bonus of Mastery experience instead – see Earning Mastery for the details:
Regular Halloween trick-or-treating, candy shopping, decorating, and cosmetic rewards do not grant Mastery Gems or experience. If you want gems, you need to be out fighting monsters.